The owlbear is one of the more dangerous low level monsters you'll face in NetHack, although usually not as lethal as those that appear in groups (like soldier ants). As the name suggests, they are a cross between a giant owl and a bear; standing upright on two legs with fur, feathers, and a beak.

Owlbears have a grabbing attack in which the owlbear bearhugs you, holding you in place and crushing you. They were introduced way back in Hack 1.0 and were probably inspired by the Dungeons & Dragons monster of the same name.

You can safely kill an owlbear from afar with ranged weapons, spells, or wands. If you are caught in the grabbing attack, you can teleport away. Using sleep or paralysis, such as from a wand of sleep or thrown potion, on the owlbear has a chance of releasing you, but not always (and if not, it also might wake the owlbear when you try to move away)[1].

Engraving Elbereth can also cause the Owlbear to flee and release you.

Owlbears are probably the crossbreed creation of a demented
wizard; given the lethal nature of this creation, it is quite
likely the wizard who created them is no longer alive. As
the name might already suggest, owlbears are a cross between
a giant owl and a bear. They are covered with fur and
feathers.